PackageManagerShellCommand: Add command to initiate a rollback.

Will be useful in future testing. In particular, for testing flows
that involve the creation and removal of users.

Test: \
  adb install -r --enable-rollback RollbackTestAppAv1.apk \
  adb install -r --enable-rollback RollbackTestAppAv2.apk \
  adb shell pm set-installer com.android.tests.rollback.testapp.A com.android.shell \
  adb shell pm rollback-app com.android.tests.rollback.testapp.A
Change-Id: I93f7ef5da8a25c8c3c4ee2c3374eb4d423fa24cb